In certain foam plastics molding, a machine, as described for example in the Applicant's EP-A-0 968 805, is known comprising a fixed frame with a window, higher than the machine supporting surface, for receiving the mold; and the mold, comprising two interlocked half-molds, is loaded onto the machine in a loading sequence comprising:                a first step of setting up and connecting a machine-mold interface frame to the mold to form a molding unit, which comprises the mold and interface frame, and has a number or quantity of plastic molding material and operating fluid inlets;        a second step of advancing the molding unit towards the fixed frame;        a third step of lifting the molding unit to window level; and        a fourth step of loading the molding unit into a work position physically and functionally engaging the fixed frame at the window, and in which the molding unit is locked to the fixed frame.        
In the work position, the mold is connected automatically to feed lines supplying granular plastic material and operating fluids, such as compressed air and steam.
In the work position, the half-molds are made movable with respect to each other by locking a first half-mold, connected to the feed lines, to a fixed frame of the machine at the window; locking a second half-mold to a movable frame of the machine facing the window; making the two half-molds free to move with respect to each other; and moving the movable frame to and from the fixed frame in a given operating sequence.
One of the main problems of known molding machines of the above type is loading the molding unit accurately onto a lift movable vertically on the machine to perform the third step described above.
In this connection, it should be appreciated that a molding unit can weigh several tons, is always off-balanced (when suspended, is always tilted with respect to the vertical), and, when attached to the lift, assumes with respect to the machine a precise final work position which, except for the movements of the molding unit on the machine at the third and fourth steps, remains substantially unchanged throughout the operating cycle of the machine. Attaching the molding unit to the lift therefore involves an indefinite number (varying from one unit to another) of setting and balancing operations, which are not only time-consuming, but must be performed with the machine off, and, in the case of medium production runs, may cut the output capacity of the machine by as much as half.